1. Field of the Invention
The present invention concerns an amplifier circuit, as can be applied for example as a function block in a microelectronic integrated circuit. In particular the invention concerns a so-called class AB amplifier circuit in which a compromise is provided between high linearity of amplification and low power consumption.
2. Description of the Prior Art
This type of amplifier circuit is elucidated, for example, in EP 1 353 440 A1.
An amplifier circuit based on a development within the business organisation of the applicant comprises:                a first supply terminal and a second supply terminal for the supply to the circuit of a first supply potential and a second supply potential,        an output stage with a first output node and a second output node for the output of a differential output signal, wherein the first output node is connected via a first output transistor with the first supply terminal, and via a second output transistor with the second supply terminal, and wherein the second output node is connected via a third output transistor with the first supply terminal and via a fourth output transistor with the second supply terminal, and        a control stage, to which an input signal can be applied to control the output transistors, for the adjustment of quiescent currents flowing through the output transistors even in the absence of the input signal, and for the adjustment of the output signal as a function of the input signal.        
An advantage of this amplifier circuit, structured according to the class AB principle, consists in that its output stage delivers a differential, and thus universally usable, output signal.
For the achievement of an amplifier characteristic that is stable in operation it has been demonstrated to be advantageous if at least one of the quiescent currents flowing through the output transistors (“class A component of the class AB amplifier stage”) is controlled by means of a feedback.
Furthermore it is advantageous for many applications if the common mode potential of the differential output signal is similarly controlled to a desired required value by a feedback.
Accordingly for the amplifier circuit described in the preceding paragraphs the attempt has been made to control both the quiescent currents and also the common mode potential to particular required values by respective feedback paths. However a serious problem ensued, in that the quiescent current control and the common mode potential control influence one another and as a result the performance characteristics of the amplifier are disadvantageously impaired.
If the bandwidths of the two control loops are similar there even exists the risk that an instability may be generated as a result of coupling between the control functions. It is in fact possible to avoid this problem by the use of very different bandwidths for the two control loops. In this case, however, there remains the disadvantage that the parameter controlled by the slower control loop is then controlled less accurately.